1. Field of the Invention
The invention generally relates to an illumination system of a microlithographic projection exposure apparatus, and in particular to such an illumination system comprising an array of micromirrors or other light deflecting elements that can be individually controlled for variably illuminating a pupil plane of the illumination system.
2. Description of Related Art
Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to light of a certain wavelength. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be imaged onto the photoresist. After exposure the photoresist is developed to produce an image that corresponds to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system that illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example. The apparatus further comprises a mask stage for aligning the mask, a projection objective (sometimes also referred to as ‘the lens’) that images the illuminated field on the mask onto the photoresist, and a wafer alignment stage for aligning the wafer coated with the photoresist.
One of the essential aims in the development of projection exposure apparatus is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to a high integration density, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus. Furthermore, with high integration densities more components can be produced on a single wafer, which has a positive effect on the throughput of the apparatus.
Various approaches have been pursued in the past to achieve this aim. One approach is to improve the illumination of the mask. Ideally, the illumination system of a projection exposure apparatus illuminates each point of the field illuminated on the mask with projection light having a well defined total energy and angular irradiance distribution. The term angular irradiance distribution describes how the total light energy of a light bundle, which converges towards a particular point on the mask, is distributed among the various directions of the rays that constitute the light bundle.
The angular irradiance distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be imaged onto the photoresist. For example, relatively large sized features may require a different angular irradiance distribution than small sized features. The most commonly used angular irradiance distributions are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil plane of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil plane. Thus there is only a small range of angles present in the angular irradiance distribution of the projection light, and all light rays impinge obliquely with similar angles onto the mask.
Different approaches are known in the art to modify the angular irradiance distribution of the projection light in the mask plane so as to achieve the desired illumination setting. For achieving maximum flexibility in producing different angular irradiance distribution in the mask plane, it has been proposed to use a spatial light modulator comprising a mirror array that produces the desired irradiance distribution in the pupil plane.
In EP 1 262 836 A1 the mirror array is realized as a micro-electromechanical system (MEMS) comprising more than 1000 microscopic mirrors. Each mirror can be tilted about two orthogonal tilt axes so that incident projection light is reflected along a direction which is determined by the tilt angles of the respective mirror. A condenser lens arranged between the mirror array and a pupil plane translates the reflection angles produced by the mirrors into locations in the pupil plane. There, or on an optical integrator which is arranged in or in close vicinity to the pupil plane, each mirror produces a light spot whose position can be varied by tilting the mirror. Each light spot is freely movable across the pupil plane or a light entrance surface of the optical integrator by tilting the respective mirror.
Similar illumination systems using mirror arrays as spatial light modulators are known from US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2.
In excimer lasers, which are usually used as light sources in the illumination system of VUV projection exposure apparatus, beam pointing fluctuations occur. This means that the direction of the light beam emitted from the laser varies to some extent in the long and/or short term. Since the light source is often arranged several meters away from the mirror array, even minute changes of the light beam direction result in significant displacements of the irradiance distribution which is produced by the projection light on the mirror array. This may ultimately lead to changes of the angular irradiance distribution in the mask plane that cannot be tolerated.
WO 2009/080279 A1 proposes to arrange an optical integrator comprising a plurality of microlenses between the light source and the mirror array. Adverse effects of beam pointing fluctuations on the stability of the angular irradiance distribution at mask level are thus avoided. However, the provision of an optical integrator significantly contributes to the costs of the illumination system and increases its complexity.